1. Field
The present invention relates to an optical amplifier including a Raman amplifier, and may be applied to an optical amplifier that uses a highly-nonlinear medium to perform Raman amplification or a method of driving the optical amplifier.
2. Description of the Related Art
A loss of signal light transmitted from a transmitter occurs in an optical communication system while the signal light propagates through a transmission path. An intensity of the signal light received by a receiver is lowered. If the intensity of the signal light received by the receiver is less than a specific value (specific threshold value), a reception error occurs, and it is difficult to transmit the signal light normally. Therefore, generally, an optical amplifier is provided between the transmitter and the receiver to amplify the signal light, thereby compensating for the loss.
The optical amplifier is a particularly important optical component in a case of long-distance transmission. Communication demands have increased in recent years with the widespread use of the Internet, and a wavelength division multiplexing (WDM) system using the wide bandwidth of an optical amplifier has been used. In addition, a WDM system having a wavelength routing function, which adds or drops signal lights by wavelength, has been used for a metro ring network together with an optical amplifier.
Examples of the optical amplifier include a rare earth-added optical fiber amplifier, a semiconductor optical amplifier (SOA), and an optical fiber Raman amplifier. Examples of rare earth elements added to the rare earth-added optical fiber amplifier include Er (erbium) that amplifies a wavelength band of 1525 nm to 1625 nm, Tm (thulium) that amplifies a wavelength band of 1480 nm to 1510 nm, and Pr (praseodymium) that amplifies a wavelength band of 1300 nm.
An Er-doped fiber amplifier (EDFA) that has generally been used in an optical communication system has a high gain characteristic, a low noise figure (NF), and a high saturation output power characteristic.
For example, a post-amplifier, a pre-amplifier, and an inline amplifier are used in an optical communication system. The post-amplifier is arranged on the output side of a transmitter, the pre-amplifier is arranged before a receiver, and the inline amplifier is provided in the case of multistage relay.
A CWDM (coarse WDM) system performs wavelength division multiplexing at a coarse interval of 20 nm in a wide wavelength band of 1470 nm to 1610 nm. As the CWDM system uses the coarse signal light interval, optical multiplexer or demultiplexer used in the CWDM between do not need less correctness about wavelength than in DWDM system and can be relatively inexpensive.
In the CWDM system, the optical amplifier can be used to transmit data over 100 km. In the CWDM system, a Raman amplifier has been used which amplifies a signal in a wavelength band (S-band) of 1470 nm which is difficult to amplify at the EDFA and is capable of freely selecting an amplification band in order to ensure a wide bandwidth, as disclosed in Japanese Laid-open Patent Publication No. 2006-074344, for example.
The Raman amplifier is an optical amplifier that uses Raman amplification in an optical fiber. The Raman amplification amplifies signal light using stimulated Raman scattering that occurs when pumping light is input to an optical fiber. The Raman amplifier is characterized in that it has a low noise figure.
For example, any of the following amplifiers is used as the Raman amplifier of the optical communication system: a distributed Raman amplifier (DRA) in which an optical fiber of a transmission path is used as an amplification medium and pumping light is input from a station to the optical fiber; and a lumped Raman amplifier (LRA) in which a highly-nonlinear optical fiber module is arranged in a station and pumping light is input to the highly-nonlinear optical fiber module. In the CWDM system, the lumped Raman amplifier (hereinafter, referred to as an LRA) capable of effectively performing Raman amplification has generally been used.
The lumped Raman amplifier uses a highly-nonlinear medium to perform Raman amplification. Therefore, various nonlinear phenomena other than Raman amplification occur, and noise light is generated in the signal light subjected to the Raman amplification. Examples of the nonlinear phenomena other than Raman amplification include self phase modulation (SPM), cross phase modulation (XPM), stimulated Brillouin scattering (SBS), and four wave mixing (FWM).
In particular, the stimulated Brillouin scattering is a nonlinear phenomenon involving wavelength (frequency) conversion, and noise light having a wavelength that is different from that of signal light is generated by the stimulated Brillouin scattering. Therefore, in the WDM system, when the stimulated Brillouin scattering occurs in the signal light, noise light leaks to adjacent channels, and signal light of the adjacent channels is deteriorated by crosstalk.
The stimulated Brillouin scattering propagates through the highly-nonlinear medium in a direction that is opposite to the propagating direction of the signal light. Therefore, the stimulated Brillouin scattering does not matter in a one-way optical communication system. In addition, a wavelength shift is of the order of 0.01 nm in a two-way optical communication system, and crosstalk does not occur in other signal light components.
However, the stimulated Brillouin scattering consecutively occurs, and the wavelength shift is increased to several tens of nanometers when a highly-nonlinear medium having a relatively short length is used to intensely perform Raman amplification as in the lumped Raman amplifier. Therefore, crosstalk occurs in the WDM system using the lumped Raman amplifier in other signal light components due to the stimulated Brillouin scattering.
FIG. 18 is a graph illustrating a spectrum of light output from the lumped Raman amplifier when the intensity of input light is low. In FIG. 18, the spectrum of output light is illustrated when light having a wavelength of 1567 nm is input to the lumped Raman amplifier with sufficiently low intensity. In this case, no stimulated Brillouin scattering occurs, and only a component 1810, which is an amplified component of input light, is output.
FIG. 19 is a graph illustrating a spectrum of light output from the lumped Raman amplifier when the intensity of input light is high. In FIG. 19, the spectrum of output light is illustrated when light having a wavelength of 1567 nm is input to the lumped Raman amplifier with an intensity that is 2.3 dBm higher than that shown in FIG. 18. In this case, stimulated Brillouin scattering occurs, and the component 1810, which is an amplified component of input light, and a noise light component 1910 generated due to the stimulated Brillouin scattering are output.
As described above, stimulated Brillouin scattering occurs when the intensity of light input to a highly-nonlinear medium is greater than a specific intensity value. Therefore, stimulated Brillouin scattering occurs in the WDM system when a new channel is added by driving a new LD (Laser Diode) and a total intensity of light input to a highly-nonlinear medium is rapidly increased, for example.
FIG. 20 is a graph illustrating light output from the lumped Raman amplifier when a channel is added. In FIG. 20, the horizontal axis indicates time, and the vertical axis indicates the intensity of light. Reference numeral 2010 denotes light input to the lumped Raman amplifier. Reference numeral 2020 denotes light output from the lumped Raman amplifier. Only a transmitter having a wavelength of 1530.8 nm is driven in the WDM system during a period T1.
A transmitter having a wavelength of 1512.9 nm is newly driven at a time t1 after the period T1. A newly added transmitter outputs continuous light during a period T2 after the time t1, without modulation. Light output from the newly added transmitter is modulated during a period T3 after the period T2.
In FIG. 20 of the conventional optical amplifier, noise light is generated in the output light 2020 due to stimulated Brillouin scattering during the period T2. In FIG. 20, noise light (reference numeral 2040) of about ±2.5 dB is generated in the output light 2020.
FIG. 21 is a graph illustrating light output from the lumped Raman amplifier when a channel is added. In FIG. 21, the same components as those shown in FIG. 20 are denoted by the same reference numerals, and a description thereof will be omitted. In FIG. 21, output light is illustrated when a transmitter having a wavelength of 1510 nm is newly added, instead of the transmitter having a wavelength of 1512.9 nm shown in FIG. 20. During a period T1, only a transmitter having a wavelength of 1530.8 nm is driven in the WDM system.
The transmitter having a wavelength of 1510 nm is newly driven at a time t1 after the period T1. The transmitter having a wavelength of 1510 nm outputs continuous light during a period T2 after the time t1. As shown in FIG. 21, noise light is continuously generated in the output light 2020 due to stimulated Brillouin scattering during the period T2.
As illustrated in FIG. 19, stimulated Brillouin scattering occurs on the longer wavelength side of signal light. By adding a channel to the longer wavelength side of the channel that is in service, the occurrence of crosstalk in other signal light components due to stimulated Brillouin scattering is prevented. However, the order in which channels are added is strictly limited.